1. Field of the Invention
This invention is related in general to the field of pelletizing and briquetting of coal fines and, in particular, to a novel approach based on the use of liquefied biomass as a reactive coal binder.
2. Description of the Related Art
In coal mining processing and handling, enormous tonnages of coal fines are created Typically, after handling and cleaning is completed, about fifteen to twenty percent of the tonnage mined consists of fines ranging in size from powder to small granules For the most part, these fines are not directly usable, thereby leaving great quantities of material that is wasted and represents a hazardous and expensive disposal problem. While a portion of the coal fines can be combined with coarser fractions of mine production for sale, the inclusion of all fines often reduces the quality of the product below market requirements Accordingly, coal-fines handling, storage and disposal operations represent a significant and unproductive expense for the industry.
A problem that contributes to the underutilization of coal fines for conventional uses is their relatively high moisture content after processing in coal preparation plants. Obviously, the price of coal depends on its heating value and is increased by reducing the moisture content. Therefore, the coal industry typically reduces the moisture in coal prior to shipment to utilities and industrial customers in order to increase its heating value and price. In addition, reducing the moisture content of coal increases the efficiency of power plants and decreases transportation costs. The process of reducing coal moisture, though, further contributes to the problem of coal fines production because conventional coal drying processes generate large amounts of coal fines, which are created as water is removed by the weakening of the coal structure and the attrition of coal particles. Thus, coal drying operations represent another source of fines that are either wasted or represent an added operating expense.
Lower-rank coals, which include lignite and some bituminous coals, are generally regarded as a low quality fuel source because of their high water and fines contents in addition to their lower Btu values. On average, pure forms of lower-rank coals contain up to about 60% moisture by weight, which causes a substantial percentage of the energy produced by these coals being used to dry them prior to full combustion. During production, because of their softness, lower-rank coals break down and produce a higher than normal volume of fines. This makes their handling extremely difficult and also hazardous because the large surface area associated with large volumes of fines results in an increased tendency for spontaneous combustion.
Despite these disadvantages, though, lower-rank coals may also provide significant advantages to industry, often in the form of low ash and low nitrogen contents, high reactivity and lower mining costs. However, in order to exploit these advantages industrially, both the problems of moisture content and fines handling must be addressed.
As a result of these problems, as well as of strict customer demands with respect of coal quality and of increasingly stringent regulation of mine waste disposal practices to satisfy environmental standards, coal fines utilization has been recently reexamined by the industry. In the past, fines have been used mostly for manufacturing briquettes for home and commercial heating. Coal briquetting technology focused on low pressure agglomeration of coal fines, using a binder, typically of coal tar origin, to hold individual particles together. This technology flourished during the early part of the century, when coal-briquette products were utilized as a home heating fuel, but this application has essentially disappeared since the end of World War II because of a shift to other, more convenient sources of fuel. Therefore, this opportunity for commercial utilization of coal fines has been drastically reduced.
The fines material from mining operations is frequently in the form of a wet filter cake containing between about twenty and thirty percent moisture, depending upon its size distribution and ash content. In a dry state, the fines are generally predominantly passable through a 28-mesh screen, a size that may be used for pelletizing and/or briquetting purposes. As used in this disclosure, the terms pellet, briquette, log and block are used interchangeably and are intended to refer to all forms of pellets, briquettes, logs, blocks and other coal agglomerates produced by binding coal fines into a concrete material. Similarly, the terms pelletizing and briquetting are intended to refer to equivalent processes for producing coal agglomerates and are also used interchangeably.
Numerous processes have been proposed and implemented in the past for pelletizing and/or briquetting particles of coal or coke. For example, U.S. Pat. No. 44,994, issued over a century ago, teaches that coal dust can be pelletized by saturating it with a solution of starch, pressing or otherwise forming it into blocks or lumps, and drying it in the sun or by other suitable means. U.S. Pat. No. 852,025 discloses preparing coal for briquetting by drying and heating it, mixing in an asphaltic binder material, then heating, cooling, and compacting the mixture. U.S. Pat. No. 1,121,325 discloses briquetting coal fines by mixing dry coal and starch, adding steam that is saturated with oil, then compressing and thermally drying the mixture. U.S. Pat. No. 1,851,689 discloses briquetting coal fines by mixing the coal with a starch/oil emulsion and then autoclaving it at 300.degree. F. U.S. Pat. No. 4,049,392 discloses an extrusion apparatus, as described in U.S. Pat. No. 3,989,433, for extruding rod-like bodies from coal-containing particulate mixtures, and having means for adjusting the length and density of the extruded particles.
The current emphasis in finding useful applications for coal fines is their utilization in combination with binders to produce pellets especially for the stoker coal markets, but also as a more convenient method of shipping to electric power plants for subsequent on-site pulverization and combustion. Thus, the stoker markets include a large industrial component that could provide an outlet for the economic and efficient consumption of this material that is currently largely wasted.
Many natural and synthetic polymers have been utilized as binders for coal fines. U.S. Pat. No. 5,244,473 teaches that a binder for coal fines can be made from a phenol-aldehyde resin mixed with a polyisocyanate in the presence of a catalyst. U.S. Pat. No. 5,089,540 teaches that a binder for foundry molds can be an ester-cured alkaline phenolic resin, which can be enhanced by conditioning the reclaimed sand with a solution containing an amine and a silane. U.S. Pat. No. 5,009,671 teaches that coal briquettes can be made by using a starch binder mixed with molasses and water. U.S. Pat. No. 4,862,485 teaches how to make coal pellets by mixing coal particles with polyvinyl alcohol, calcium oxide and/or magnesium oxide and water. U.S. Pat. No. 4,738,685 teaches how to cold press coal fines with molasses, an inorganic hardening agent such as calcium carbonate, calcium phosphate, iron oxide, aluminum oxide and optionally with an acid. U.S. Pat. No. 4,618,347 teaches how to make coal pellets from coal dust and a binder consisting of lignosulfonate plus sodium dichromate, while using sulfuric acid as a curing agent. U.S. Pat. No. 4,586,936 shows how to make coal pellets from lower rank coal mixed with cationic polyurethane and polyvinyl alcohol. U.S. Pat. No. 4,169,711 discloses that coal particles (1/4 to 3/4 inch) mixed with coal fines can be briquetted into `smokeless` fuel logs when mixed with sodium silicate and/or potassium silicate. Finally, U.S. Pat. No. 3,966,427 teaches how to make coal briquettes using bitumen or bitumen emulsions as binders.
Many prior-art binders use water in the process of mixing with coal fines to produce briquettes, thereby further increasing the moisture content of the product. Thus, for example, when starch-based binders are used, the resulting green pellets must be dried to achieve acceptable fuel performance and reduce transportation costs. In addition, prior-art binders are derived from useful and often expensive raw materials, such as natural and synthetic polymers; therefore, they add significantly to the overall cost of the briquette. Finally, known binders perform an adhesive function by physically binding the coal particles together to form a larger mass; no chemical reaction which would strengthen the bond is understood to take place between the binder and the coal particles.
Therefore, there is still a need for improved binders and briquetting processes. The present invention is based on the discovery that biomass waste materials can be effectively utilized to produce a coal-fines binder that represents a significant improvement over the properties of the binders used to date.